Integrated systems tend to have several subsystems and power sections integrated on a single chip. These devices commonly use different positive power supplies, and often use negative power supplies to power in the most efficient manner the different chip sections or the various subsystems integrated on the same chip.
Sometimes one or more supply voltage sources are externally provided, while others are implemented internally by as many voltage regulators as there are externally generated supply voltages. In these monolithic integrated complex systems, it is very important to monitor the level of all the different supply voltages and to generate a so-called NPOR signal stating their compliance or noncompliance with their respective design values.
For an integrated system with one or more regulators generating as many supply voltages within the integrated circuit, it may be crucial to verify that all the supply voltages that are either fed to the IC or generated internally have reached pre-established values before enabling functioning of the integrated circuit.
Conventionally, this verification is acknowledged at the end of the charging process via a capacitor charged with a relatively low constant current to prevent the functional circuits from producing false switchings because of an insufficient supply voltage at the turn-on of the IC. Because the capacitor must be charged within a few milliseconds, it must have a relatively high capacitive value. This capacitor is commonly an external capacitor connected to a dedicated pin of the IC.
The assertion that all the supplies of the functional circuits of the integrated circuit have reached their pre-established values is done conventionally by forcing the NPOR signal to a high logic level. On the other hand, many electronic systems implement a turn-on method that is commonly referred to as soft start to avoid an abrupt application of the full supply voltages to the IC. Even for this well known soft start function, a relatively slow charging external capacitor having a large capacitance is the common way of implementation.
It is evident the burden implied by the use of distinct external capacitors, one for the soft start function and the other for the power monitor function. Each capacitor respectively requires a dedicated pin on the IC.